1. Technical Field
The present invention relates to an illumination lens disposed in a front end of a light guide of an endoscope, and an endoscope provided with the illumination lens disposed in a front end of a light guide.
2. Related Art
In the background art, an illumination optical system of an endoscope is typically constituted by a concave lens disposed in a front end of a light guide for guiding illumination light, so that the illumination light emitted from the light guide can be refracted outward and diffused. With such a configuration, light rays are kicked on the side face of the lens. Thus, loss in the quantity of light is comparatively large in the periphery of an irradiation field.
There has been therefore a proposal that a lens whose entrance face is formed as a convex face with positive power is disposed in a front end of a light guide (for example, see JP-A-2000-193894). With such a configuration, illumination light emitted from the light guide is once collected (refracted inward) by the lens and then diffused so that light rays can be prevented from being kicked on the side face of the lens. Thus, improvement in utilization efficiency of the illumination light can be expected.
In view of the design of the lens, however, there is a limit on wide-angle light distribution which can be obtained by the lens whose entrance face is formed as a convex face with positive power, even when the entrance face is made aspherical. The entrance face of the illumination lens described in JP-A-2000-193894 is formed as a light diffusing face in order to cancel unevenness in light distribution. According to JP-A-2000-193894, wider-angle light distribution can be expected by a diffusion effect while unevenness in the light distribution can be cancelled. However, there is a fear that the utilization efficiency of the illumination light improved by the entrance face formed as a convex face with positive power may deteriorate due to the diffusion.
In recent years, heat capacity of an endoscope has decreased with reduction in the diameter of the endoscope, and power consumption of the endoscope has increased with increase in the number of pixels in an image sensor mounted in the endoscope. It has been therefore indispensable to take measures to suppress temperature rise in a front portion of an insertion portion. Illumination light also becomes a heat source. However, the quantity of illumination light can be suppressed if the utilization efficiency of the illumination light can be improved. Thus, heat caused by the illumination light can be cut off to suppress temperature rise in the front end portion of the insertion portion.
The invention was developed in consideration of the aforementioned circumstances. An object of the invention is to improve the utilization efficiency of illumination light and achieve wider-angle light distribution in an illumination lens disposed in a front end of a light guide of an endoscope.